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From The Collaborative International Dictionary of English v.0.48:
Thallogen \Thal"lo*gen\, n. [Gr. ? young shoot or branch, frond + -gen.] (Bot.) One of a large class or division of the vegetable kingdom, which includes those flowerless plants, such as fungi, algae, and lichens, that consist of a thallus only, composed of cellular tissue, or of a congeries of cells, or even of separate cells, and never show a distinction into root, stem, and leaf. [1913 Webster]